The disclosure of Japanese Patent Application No. 11-296176 filed on Oct. 19, 1999, including the specification, drawings and abstract is incorporated herein by reference in its entirety.
1. Field of the Invention
The invention relates to an electric power steering unit for a vehicle, which assists steering operations of a steering wheel by the rotating operation of an electric motor and to methods for controlling the steering unit.
2. Description of Related Art
An electric power steering unit for a vehicle is disclosed in Japanes Patent Application Laid-Open No. 10-157636, which applies an assisting force with respect to the rotating operation of a steering wheel by controlling the rotational force of an electric motor in accordance with the detected steering torque. This unit serves to compensate for insufficiency of the assisting force caused by an inertia force of the electric motor at the start of a rotating operation of the steering wheel or the steering torque due to friction. This can be done by adding controlled amount, corresponding to the differential value differentiated by the steering torque, to the steering torque that otherwise would be provided.
In the aforementioned electric power steering unit, the faster the steering wheel is rotatively operated, the more the steering assisting force by the electric motor is increased because of an increase in the differential value of the steering torque. Accordingly, the driver may receive substantially different reaction forces with respect to a steering operation depending on whether the steering wheel is rotated at low speeds or at high speeds. Such difference in the steering reaction force received by the driver may deteriorate the steering feel for the driver. A reverse rotating operation of the steering wheel may generate a state where two control amounts corresponding to the steering torque and the differential value of the steering torque have opposite signs, each acting in opposite directions. Therefore the driver does not feel the reaction force which is expected to be felt from a normal steering operation of the steering wheel, or the operated steering wheel may return to the original position rapidly. In this case as well, good steering feel may not be obtained.
The invention has been made in view of the foregoing disadvantages of the related art. It is an object of the invention to provide an electric power steering unit that maintains good steering feel for the driver. The electric power steering unit according to a first aspect of the invention is provided with an electric motor that applies an assisting force with respect to a rotating operation of a steering wheel, a steering torque detector that detects a steering torque, and a controller that generates an initial inertia compensation control amount to offset the inertia force of the electric motor. The controller controls the rotational force of the electric motor based on the detected steering torque and a corrected initial inertia compensation control amount. The controller corrects the initial inertia compensation control amount in accordance with a detected steering speed of the steering wheel.
According to the first aspect of the invention, the controller corrects the initial inertia compensation control amount to offset the inertia force of the electric motor in accordance with the steering speed of the steering wheel that has been detected by a steering speed detector. This controller can, for example, correct the initial inertia compensation control amount to decrease with an increase in the detected steering speed of the steering wheel. This makes it possible to solve the problem of deterioration of the steering feel resulting from a variable rotating speed of the steering wheel, thus maintaining good steering feel for the driver.
In the above case, the steering speed of the steering wheel may be detected in accordance with outputs from a sensor that detects the rotating angle of the steering wheel, which is attached to the steering shaft. Alternatively, it is also possible to obtain the steering speed by calculation based on the terminal voltage of the electric motor that rotates integrally with the steering wheel and the drive current for the electric motor. According to this method, the aforementioned sensor can be omitted, thus reducing the cost for manufacturing the electric power steering unit.
An electric power steering unit according to a second aspect of the invention includes a controller that limits the inertia compensation control amount when the detected steering torque and the inertia compensation control amount are acting in opposite directions. According to this aspect of the invention, the steering torque and the inertia compensation control amount will never offset each other during a reverse steering operation of the steering wheel. Therefore, the driver never feels a lack of reaction force from a steering operation of the steering wheel, and rapid motion of the returning steering wheel is prevented, thus maintaining good steering feel for the driver.